1. Field of the Invention
The present invention relates to an extreme ultraviolet (EUV) light source apparatus for generating extreme ultraviolet light by applying a laser beam to a target material to turn the target material into plasma, and specifically, to an EUV light source apparatus for supplying high-quality extreme ultraviolet light with spectrum purity improved by eliminating the influence of the laser beam applied to the target material.
2. Description of a Related Art
Recent years, as semiconductor processes become finer, photolithography has been making rapid progress to finer fabrication. In the next generation, microfabrication at 70 nm through 45 nm, further, microfabrication at 32 nm and beyond will be required. Accordingly, in order to fulfill the requirement for microfabrication at 32 nm and beyond, for example, exposure equipment is expected to be developed by combining an EUV light source generating EUV light having a wavelength of about 13 nm and reduced projection reflective optics.
As the EUV light source, there is an LPP (laser produced plasma) light source using plasma generated by applying a laser beam to a target (hereinafter, also referred to as “LPP type EUV light source apparatus”).
In the LPP type EUV light source apparatus, a target material is injected from a nozzle and a laser beam is applied toward the target material, and thereby, the target material is excited and turned into plasma. Various wavelength components including extreme ultraviolet (EUV) light are radiated from the plasma.
Accordingly, a desired EUV light is selected using a collector mirror (EUV collector mirror) for selectively reflecting and collecting a desired wavelength, and the desired EUV light is output to external equipment such as an exposure unit. For example, when EUV light having a wavelength near 13.5 nm is collected, an EUV collector mirror having a reflecting surface is used on which a multilayer film with alternately stacked molybdenum and silicon (Mo/Si multilayer film) is formed.
However, also the light directly radiated from the target plasma and excitation laser beam reflected from the target and so on are mixed in the desired EUV light. A resist for exposure to be used in the EUV exposure unit is exposed to light having a wavelength from 130 nm to 400 nm in the spectrum of light generated from the target plasma, and it may reduce the exposure contrast. Further, infrared light contained in the excitation laser beam may be absorbed by optical parts, masks, wafers, and so on, to cause thermal expansion, and it may reduce the accuracy of patterning. Therefore, it is necessary to suppress those light components.
Conventionally, in the LPP type EUV light source apparatus, a spectrum purity filter (SPF) has been used for removing components unnecessary for EUV exposure from the spectrum of light radiated from plasma. In a technology disclosed in U.S. Pat. No. 6,809,327 B2, as shown in FIG. 17, a laser beam emitted from a carbon dioxide (CO2) laser is introduced into a vacuum chamber and focused, the laser beam is applied to a target of tin (Sn) droplets or the like supplied by a target supply unit to turn the target into plasma, light radiated from the plasma is collected by an EUV collector mirror, and the collected light is spectrum-separated by a grating type SPF, and thereby, only the EUV light having a wavelength around 13.5 nm (negative first-order light in the drawing) is guided to an exposure unit.
Further, by providing a thin film filter between the exposure unit and the vacuum chamber, Sn debris flying from the target material (Sn) introduced into the vacuum chamber and the target plasma is prevented from flowing to the exposure unit side and contaminating optical parts within the exposure unit. When a material such as zirconium (Zr) or silicon (Si) with higher transmittance for EUV light having a wavelength around 13.5 nm than for other wavelengths is selected, the thin film filter also serves as a thin film filter type SPF. In the conventional LPP type EUV light source apparatus as shown in FIG. 17, the light spectrum-separated and eliminated by the grating type SPF is absorbed by a dumper and turns into thermal energy.
Further, Japanese Patent Application Publication JP-P2006-191090A discloses another SPF using apertures or an aperture array for reflecting light having a longer wavelength than twice the width of the aperture to suppress transmission of the light.
Furthermore, Japanese Patent Application Publication JP-P2007-129209A discloses using, as an SPF, a gas curtain formed by combining necessary kinds of gases that do not have absorption capability for EUV light but have absorption capability for wavelengths to be eliminated.
Especially, in the LPP type EUV light source apparatus using a CO2 laser beam (infrared light having a wavelength of 10.6 μm) for excitation of the Sn target, the CO2 laser beam having high-power is also reflected or scattered by the target or the like, and it is necessary to remove the CO2 laser beam by the SPF. For example, assuming that the intensity of the EUV light with the center wavelength of 13.5 nm is “1”, the intensity of the CO2 laser beam is required to be suppressed to about “0.1” or less. Accordingly, in view of removal of the CO2 laser beam, there have been the following problems in the above-mentioned conventional technologies.
(1) Since the transmittance of the thin film filter type SPF that isolates the exposure unit from the EUV light source apparatus is as low as about 40%, the output efficiency of EUV light is very poor. Further, the thin film is easily broken by the incidence of debris. Furthermore, when debris adheres to the thin film, the debris absorbs EUV light and the temperature rises, and the filter itself may be melted, and therefore, it is difficult to maintain the function as the SPF.(2) In the SPF using an aperture array, there are issues of improving the efficiency of EUV light to be outputted to the exposure unit by improving the aperture ratio while maintaining the structural strength of the SPF, improving the reflectance of the CO2 laser beam to be blocked, and reducing the risk of deformation and breakage due to temperature rise caused by light absorption. Further, the fine intensity distribution of EUV light generated in the aperture array may disturb the exposure uniformity of the semiconductor and cause exposure variations.(3) In the SPF utilizing the selective absorption of gases, no kinds of gases suitable for absorption of CO2 laser beam is disclosed.